THE PRESENT INVENTION relates to a vehicle impact sensor arrangement and more particularly relates to a vehicle impact sensor arrangement adapted to sense an impact on a vehicle and to activate a safety device within the vehicle such as an air-bag or a seat belt pre-tensioner.
When a vehicle, such as a motor car, is involved in an accident, if the vehicle is subjected to an impact or collision, the vehicle can decelerate rapidly. In such a situation a person travelling within the vehicle may continue to move at the original speed of the vehicle, due to inertia and may thus impact with part of the vehicle which has decelerated. For example, if a vehicle is subjected to a front impact, by crashing into a fixed object, the main body of the vehicle may stop relatively rapidly, whilst a person in the vehicle continues to travel forwardly, the person travelling in the vehicle thus, in effect, being thrown forwardly on to a fixed part of the vehicle, such as the steering wheel or the dash board. A similar situation exists if a vehicle is subjected to a side impact.
It has thus been proposed to provide sensors which sense an impact or collision or rapid deceleration of a vehicle, and which activate safety devices such as air-bags or seat belt pre-tensioners.
Many sensors have been proposed previously, and a significant proportion of these sensors operate to provide an electrical signal responsive of the impact. The electrical signal is utilised to trigger the air-bag or the seat belt pre-tensioner. One problem that exists where an electrical triggering system is used is that sometimes a totally "spurious" signal can be generated, due to "noise" or due to external electro-magnetic radiation, which means that the air-bag or seat belt pre-tensioner can be operated when there is no collision or impact. If the car is being driven at the time, this can seriously distract the driver, and indeed, if an air-bag is inflated, the air-bag may well impair the vision of the driver.
A further disadvantage of sensors which provide a "electric" signal is that it is often the case that during a major impact the electrical supply of the vehicle may be impaired very shortly after commencement of the impact. Thus, such sensors may fail to operate satisfactorily in a real accident situation.
During an impact, the outer skin of the vehicle moves relative to a fixed inner part of the vehicle, the chassis of the vehicle. The severity of the impact is related to the speed with which the outer skin of the vehicle moves relative to a fixed part of the vehicle. It is thus desired to be able to provide a sensor which responds to the speed of movement of the outer skin of the vehicle relative to a fixed part of the vehicle, particularly in the case of a sensor adapted and located to detect a side impact.